1. Field of the Invention
The present invention relates to a multiplexer, and more specifically to a multiplexer including a plurality of filters, and to a radio-frequency (RF) front-end module including the multiplexer.
2. Description of the Related Art
Mobile phones in recent years are required to individually cope with a plurality of frequency bands and with a plurality of wireless systems, that is, multi-bands and multi-modes. To cope with these requirements, a multiplexer that divides an RF signal with a plurality of wireless carrier frequencies is disposed immediately below one antenna. As one of a plurality of band-pass filters defining the multiplexer, for example, an elastic wave filter with low loss characteristics within the passband and steep passband characteristics around the passband is used.
An exemplary elastic wave filter includes a longitudinally-coupled elastic wave filter disclosed in International Publication No. WO 2013/069225, which includes five inter-digital transducers (IDTs). In this elastic wave filter, the five IDTs are arranged along an elastic wave propagating direction on a piezoelectric substrate.
In the elastic wave filter disclosed in International Publication No. WO 2013/069225, among the five IDTs, a first IDT electrode, a third IDT electrode, and a fifth IDT electrode are commonly connected to an input-side terminal (unbalanced terminal), and a second IDT electrode and a fourth IDT electrode are connected to a different output-side terminal (balanced terminal). In paragraph [0025] of International Publication No. WO 2013/069225, the pitch of electrode fingers of the first and fifth IDT electrodes is about 1.0582 μm, the pitch of electrode fingers of the second and fourth IDT electrodes is about 1.0569 μm, and the pitch of electrode fingers of the third IDT electrode is about 1.0612 μm. That is, an IDT electrode that has the maximum electrode finger pitch (the third IDT electrode in the case of International Publication No. WO 2013/069225) is connected to the input-side terminal.